When it is necessary to increase the power output of a power amplifier, a conventional approach is to add more amplifications stages, or transistors, in parallel. This is particularly a problem in solid state amplifiers at microwave frequencies commonly achieved through GaAs field effect transistors (FETs). For example, for a typical 12 mm S-band power device with gate widths of 300 .mu.m, four individual FET amplification stages or cells may be needed to be paralleled to provide the proper output power. There are a number of shortcomings associated with such increased paralleling, which is particularly critical in the microwave range. Paralleling increases the transverse dimension of the overall amplifier, and as this dimension becomes a significant fraction of the wavelength at the frequency of interest, all of the stages cannot be excited in phase. This out of phase combination of signals through the various stages or transistors results in an overall gain reduction. Paralleling also reduces both the input and the output impedances, so that matching to the outside circuits with the available circuit elements may not be possible over the desired band width. Thus the requirement of presenting proper complex impedances to the outside devices for maximum power transfer can only be satisfied at a very narrow band of frequencies. This results in reduced band width and increased sensitivity to device and circuit parameters, and even if impedance matching is accomplished at these low impedance levels, circuit losses in the tuning elements become significant and reduce the power output and efficiency of the amplifier. Another shortcoming of paralleling transistors is that the grounding inductances become significant in comparison to reduced internal resistances and introduces a series feedback path which creates instability in larger devices, that is, those with large numbers of parallel transistors.
One of the primary problems with paralleling stages or transistors to increase power output is that it demands a proportionate increase in the d.c. current that needs to be supplied to the amplifier. In thin film circuitry used for monolithic microwave integrated circuit (MMIC) chips, components must then carry current at or above the electromigration current density, and this introduces serious long-term reliability problems. In addition, in a circuit in which packing density is high, the distribution of the required amperes of current to each module is a significant problem in itself.
These shortcomings seriously limit the maximum power output of any amplifier, and especially that from large periphery FET circuits.
There are additional problems in wide-band applications using traveling wave amplifier approaches, such as the increased loading on the gate and drain lines and the maximum allowable drain voltage swing that sets an upper limit to the usable total periphery per amplifier stage.